Conventionally, it is known that non-linear signal distortion is generated in an analog circuit, an RF (Radio Frequency) circuit and the like making up a transmitting system such as a radio communication device. To compensate for such signal distortion, a technique called adaptive digital predistortion has been developed.
In this technique, the reverse characteristics of an analog circuit and an RF circuit are stored in a LUT (Look Up Table) in the form of compensation coefficients in accordance with the amplitude and the power of the input signal. With this configuration, distortion compensation is achieved by preliminarily multiplying an undistorted baseband signal by a compensation coefficient corresponding to the amplitude and the power of the input signal.
In addition, in this technique, the input signal and the transmission signal multiplied by the compensation coefficients are compared with each other, and the compensation coefficients are adaptively updated such that the difference between the input signal and the transmission signal is reduced. In this manner, even in the case where the distortion characteristics are changed under the influence of aging change, temperature change, voltage change and the like, compensation of signal distortion is effectively executed.
For example, PTL 1 discloses a technique in which a compensation coefficient for compensating for signal distortion is generated by an adaptive algorithm based on a difference between an input signal and an output signal of an amplifier, and an input signal is multiplied by the compensation coefficient thus generated, whereby signal distortion generated by the amplifier is compensated.